In principle there are two main types of wind turbines in view of the direct drive configuration of a wind turbine. The first type of a wind turbine is the more classical type of a wind turbine comprising a gearbox arranged between a main shaft and a generator of the wind turbine. The second type of a wind turbine is a gearless type, where the gearbox and the conventional generator are substituted by a multipolar generator, a so called direct drive or directly driven generator. Such a direct drive generator can be made as a synchronous generator with winded rotor or with permanent magnets attached to the rotor, or it can be designed as an alternative type of a generator. One of the challenges with the direct drive generator is the mechanical brake system. The brake system needs to be located at the slowing rotating axis as no gear box is used. The brake system needs to withstand a large braking moment and large brake discs and callipers are necessary.
The US 2005/230979 describes a wind turbine which minimizes the size of the nacelle while providing adequate accessibility to components during maintenance operations. The wind turbine comprises: a nacelle and a blade rotor hub adjacent to said nacelle and a main shaft coupled to said hub and said nacelle. The generator is coupled to said shaft between said nacelle and said hub, wherein said generator includes generator rotor adjacent to said shaft, a stator positioned adjacent to and radially outward from said generator rotor; and a brake coupled to said generator and said shaft where the brake is positioned radially inward from said stator. The invention describes a solution that minimizes the brake system as the system can be more or less integrated into the generator. However, the proposed solution minimizes the size of the construction and also restricts the size of the brake system which is unwanted for a direct drive wind turbine where the size of the callipers and the brake disc should be as big as possible in order to withstand the high braking moments due to the slow rotational speed of the rotor. The brake system shown in US 2005/230979 also prevents passage though the generator and it is not possible to go into the hub from the nacelle/generator. Furthermore, the invention relates to a generator with an inner rotor configuration and it not possible to use the configuration for brake system with outer rotor configuration.
In US 2009/0026771 A1 a wind turbine comprising an electrical generator that includes a rotor assembly is disclosed. The wind turbine includes a frictional braking system for slowing, stopping or keeping stopped the rotation of the wind rotor and rotor assembly. In one implementation, the wind turbine/generator set includes a generator that includes a stator assembly and a rotor assembly rotatable about a rotational axis. The rotor assembly includes an active portion and an active portion support supporting the active portion. The wind turbine/generator set further includes a wind rotor coupled to the rotor assembly, a generator support fixedly supporting the stator assembly and rotatable supporting the rotor assembly. Furthermore, the wind turbine/generator set includes at least braking device fixed relative to the generator support. The braking device includes at least one frictional member operatively configured for frictionally engaging at least a portion of the active portion support. The rotor assembly is generally located radially inward of the stator assembly.